Refrigeration system with hot gas defrosting

ABSTRACT

A refrigeration system particularly adapted for supermarkets and the like employing a multiplicity of refrigerated fixtures wherein centrally located compressors operate all the evaporators and each of the evaporators may be selectively defrosted with hot gaseous refrigerant from the high-pressure side of the system. In the various embodiments a chamber of reduced pressure is provided for receiving the defrosting refrigerant after passing through the evaporator and the chamber serves to separate the gaseous and liquid refrigerant with the liquid refrigerant being returned to the normal supply of liquid refrigerant for the evaporators and the gaseous refrigerant being returned to the compressor suction in a controlled manner. In the preferred and two alternate embodiments the pressure reduction is in the liquid supply line to the evaporators with the chamber also serving to separate flash gas caused by such pressure reduction as well as the returning gaseous defrosting refrigerant while in the other embodiment the pressure reduction in the chamber does not change the liquid supply line pressure to the evaporators.

United States Patent Quick 1451 Feb. 29, 1972 [54] REFRIGERATION SYSTEM WITH HOT GAS DEFROSTING Lester K. Quick, 868 Westview Crescent, North Vancouver, British Columbia, Canada [22] Filed: Mar. 16,1970

21 Appl.No.: 19,80l

[72] Inventor:

Primary Examiner-Meyer Perlin AttorneyLy0n 8!. Lyon [57] ABSTRACT A refrigeration system particularly adapted for supermarkets and the like employing a multiplicity of refrigerated fixtures wherein centrally located compressors operate all the evaporators and each of the evaporators may be selectively defrosted with hot gaseous refrigerant from the high-pressure side of the system. In the various embodiments a chamber of reduced pressure is provided for receiving the defrosting refrigerant after passing through the evaporator and the chamber serves to separate the gaseous and liquid refrigerant with the liquid refrigerant being returned to the normal supply of liquid refrigerant for the evaporators and the gaseous refrigerant being returned to the compressor suction in a controlled manner. In the preferred and two alternate embodimerits the pressure reduction is in the liquid supply line to the evaporator-s with the chamber also serving to separate flash gas caused by such pressure reduction as well as the returning gaseous defrosting refrigerant while in the other embodiment the pressure reduction in the chamber does not change the liquid supply line pressure to the evaporators.

26 Claims, 5 Drawing Figures PATENIEBFEB29|912 3.645.109

sumanra INVENIOR 66576516 QUICK REFRIGERATION SYSTEM WITH HOT GAS DEFROSTING This invention relates to refrigeration systems for operating multiple refrigerated fixtures. such as in supermarkets, and in particular is directed to improvements in the system for efficiently and effectively accomplishing hot gas defrosting of the fixture evaporators.

In recent years there has been a trend toward the use of relatively large compressors centrally located in a supermarket installation for operating the refrigerated fixtures throughout the store. The plurality of evaporators operated by a single compressor or plural compressors combined in a parallel or compounded relationship produce a substantial heat load capable of producing the desired rapid and efficient hot gas defrosting of a limited proportion of those evaporators at any one time. This has led to an increased popularity in the use of hot gas defrosting in supermarket refrigeration systems in preference to other slower or more costly arrangements such as electrical defrosting or off-cycle defrosting.

While the advantages of employing hot gas defrosting in the central refrigeration systems of the type used in supermarkets is now rather well recognized, there has remained a number of disadvantages or problems with the heretofore developed systems. Among the most prominent problems has been the manner in which the refrigerant used in defrosting an evaporator is reintroduced into the normal refrigeration cycle with the least possible difficulty, expense and inefficiency. A number of methods for accomplishing this objective are possible, some of which are those shown in my following prior U.S. Pats. Nos. 3,151,470 entitled Hot Gas Defrosting System." 3,234,748 entitled "Hot Gas Refrigeration Defrosting System With Purge Means, 3,234,752 entitled "Desuperheater for Refrigeration System," 3,234,753 entitled Hot Gas Refrigeration Defrosting System and 3,234,754 entitled "Reevaporator System for Hot Gas Refrigeration Defrosting Systems. In each of these prior patented systems and other systems of which I am aware there were specific advantages and disadvantages from system to system but there were also common disadvantages.

One common disadvantage of hot gas defrosting systems wherein the defrosting refrigerant was reintroduced into the liquid refrigerant supply for the evaporators was the presence of an indeterminate amount of gaseous refrigerant which, on occasion, would cause the malfunctioning of one or more evaporators that were not being defrosted by supplying gaseous refrigerant thereto. The amount of gaseous refrigerant returned with the defrosting refrigerant into the liquid refrigerant supply line in these prior systems was uncontrolla ble due to such factors as changing ambient conditions, differences in frost buildup, differences in the refrigeration system loads, etc. On the other hand the liquid refrigerant returned from defrosting an evaporator as capable of and did serve a useful refrigerating function in contrast to arrangements where the liquid refrigerant was merely heated and reevaporated for recompressing without performing any useful function.

Another difficulty that has plagued central refrigeration systems of the type used in supermarkets employing hot gas defrosting is the substantial variations in operating pressures throughout different portions of the system by reason of variations in ambient and load conditions. For example it is well known that functional characteristics of conventional expansion valves used on evaporators is dependent on the pressure differential across such valves yet the pressure in the liquid refrigerant supply line to the expansion valves may vary substantially with the highest pressure being perhaps more than double the lowest pressure. This pressure variation is principally due to ambient conditions since the condensing temperature and pressure will be substantially lower in winter than in summer. This variation in condensing or head pressure also normally produces a corresponding variation in the temperature of the gaseous refrigerant used for the hot gas defrosting whereby the lower gaseous refrigerant temperatures in the winter time will produce slower defrosting of any given evaporator. Since it is conventional to control the hot gas defrosting of evaporators by predetermined time periods, it is common to experience less than complete defrosting in winter with the lower temperature gaseous refrigerant or even excessive defrosting and heating in summer.

Accordingly, a summary of a principal object of this invention is to provide an improved refrigeration system with hot gas defrosting wherein a pressure differential is affirmatively created across each evaporator that is being defrosted for providing an incentive for the flow of defrosting refrigerant therethrough and a vessel or chamber is provided for separat' ing the gaseous and liquid phases of refrigerant returned from defrosting an evaporator with the liquid refrigerant being used in the normal supply of liquid refrigerant and the gaseous refrigerant being removed and recompressed.

An object of this invention is to provide a novel arrangement in a refrigeration system for efficiently disposing of the condensate refrigerant produced in hot gas defrosting without limitation as to the proportions of liquid and gaseous refrigerant present in such condensate.

Another object of this invention is to provide a novel refrigeration system with hot gas defrosting wherein a predetermined reduction in the pressure of the liquid refrigerant being supplied to the evaporators is caused for producing the pressure differential necessary for the flow of hot gas defrosting refrigerant through selected evaporators into such liquid refrigerant supply and wherein the flash gas produced by such liquid line pressure reduction and any gase ous refrigerant returning with the condensed defrosting refrigerant are separated and controllably removed to preclude any such gaseous refrigerant from entering an evaporator during normal refrigeration operation.

Another object of this invention is to provide a novel refrigeration system with hot gas defrosting wherein the supply of liquid refrigerant to the evaporators is maintained at a predetermined constant pressure by reducing the variable condensing or head pressure to a predetermined low level and wherein the flash gas produced by such pressure reduction is continually removed by controlled return to the compressor suction. Another object of such a system is to permit the head pressure to drop to low levels thereby reducing the power consumption of the system. A still further object is to provide such a system wherein gaseous refrigerant returning with the condensate from hot gas defrosting of evaporators is also continually removed and returned to the compressor suction. A still more detailed object of this invention is to provide such a system wherein a novel form of refrigerant receiver for the entire system serves as the chamber for separating liquid and gaseous refrigerant with such gaseous refrigerant being returned from the receiver to the compressor suction in a controlled manner.

A still further object of this invention is to provide a novel refrigeration system with hot gas defrosting wherein a plurality of centrally located compressors operate a multiplicity of refrigerated fixtures at various evaporator suction pressures wherein the discharge of one or more selected compressors is employed for supplying the hot gaseous refrigerant for defrosting with the pressure of such discharge controlled to a predetermined level independent of the ambient temperature for supplying gaseous refrigerant to the evaporators selected for defrosting. Still another object is to provide such a system wherein pressure in the liquid refrigerant supply line to the evaporators is regulated to a level substantially below such preselected hot gas defrosting pressure and wherein an intermediate pressure is maintained in a header returning the defrosting refrigerant from the defrosted evaporators to the liquid refrigerant supply line.

Another object of this invention is to provide a novel central refrigeration system wherein the normal liquid refrigerant supply is greatly reduced in pressure and temperature at the location of the compressors with the flash gas produced by such reduction being directly and efficiently returned to such compressors. A further object is to provide such a system wherein the lengthy refrigerant supply and suction lines to the remote fixtures may be reduced in size by reason of such direct flash gas return. A still further object is to provide such a system wherein the flash gas is returned only to compressors operating at higher suction pressures thereby improving the efficiency of the other compressors.

Other and more detailed objects and advantages of this invention will appear from the accompanying description and drawings wherein:

FIG. 1 is a schematic illustration of the preferred form of my invention illustrating a central refrigeration system employing compounded compressors operating at three suction pressures and an air-conditioning compressor together with the hot gas defrosting arrangement.

FIG. 2 is a schematic illustration of a modified from of the refrigeration system with hot gas defrosting shown in FIG. I but with substantial portions of the overall system omitted for simplicity of illustration.

FIG. 3 is a sectional elevation view of the refrigerant receiver illustrated in FIG. 2 taken substantially on the line 3-3 in FIG. 2.

FIG. 4 is a schematic illustration of a modified form of the refrigeration system with hot gas defrosting of this invention wherein only a pressure reduction on the liquid refrigerant supply line is employed with the flash gas from such pressure reduction and the gaseous refrigerant returned from defrosting both being separated and returned.

FIG. 5 is a schematic illustration of another modified form of the refrigeration system with hot gas defrosting of this invention wherein a pressure reduction only on the header returning defrosting refrigerant is employed and only gaseous refrigerant returned during defrosting is separated and returned to the compressor.

While FIG. 1 illustrates a virtually complete system employ ing multiple compressors and FIGS. 2, 4 and 5 illustrate only single compressors and less details. it is to be understood and will readily appear from the following descriptions that any of the illustrated systems may be used with one or multiple compressors and certain components and features of each illustrated system may be applicable to the other systems. In particular for clarity and simplicity of description FIGS. 2, 4 and 5 have been illustrated in very simplified form rather than including the many components illustrated in FIG. 1 or that would be included in any such refrigeration system.

Referring now more particularly to FIG. 1, a typical refrigeration system of this invention is illustrated for accomplishing all of the refrigeration requirements in the usual supermarket as well as providing air conditioning. In the usual supermarket there are three distinct levels of refrigerated fixture temperatures required which are normally referred to as standard temperature fixtures (i.e., diary, produce and fresh meat), low temperature fixtures (i.e., frozen foods and walk-in freezers) and ultralow temperature fixtures (i.e., ice cream cases and special frozen food fixtures requiring very low back pressures). To provide these three temperature levels in a su' permarket or any installation having similar requirements, it is necessary to provide compressors operating at three distinct suction pressures. if the best operating efficiency is to be maintained, which compressors are commonly referred to by the corresponding temperature level of fixtures to which their suctions are connected. Thus in FIG. 1 there are two compressors l0 and II connected in parallel to operate the standard temperature fixtures and will be referred to as the standard temperature compressors. Two more compressors l2 and 13 are connected in parallel to operate the low temperature fixtures and compressor 14 is the ultralow temperature compressor. In addition, FIG. 1 illustrates and air conditioning system having air-conditioning compressor I5.

In their normal refrigeration operation compressors l0, 11, I2, 13 and 14 are compounded. The discharge I6 of ultralow temperature compressor 14 is connected through conduit 17 to the suction side 18 of low-temperature compressors l2 and I3. The discharge sides 19 of low-temperature compressors I2 and 13 are connected through conduit 20 to the suction side 21 of standard temperature compressors l0 and II. The discharges 22 of standard temperature compressors l0 and II are separately connected to a discharge accumulator and oil separator 23 which has an outlet standpipe 24 connected to a conduit 25 leading to the system condenser 26 and thence through a conduit 27 to the system receiver 28. In this manner the entire system has a single condenser-receiver arrangement for supplying the liquid refrigerant requirements of all of the evaporators rather than separate condenser-receivers for each of the temperature levels. The ultralow temperature compressor 14 is compounded with the low-temperature compressors I2 and 13 which are in turn compounded with the standard temperature compressors l0 and 11.

Although it forms no part of this invention it may be seen that the air conditioning system is completely compatible with the refrigeration system of this invention. The air-conditioning compressor 15 has its discharge 29 connected to the discharge accumulator and oil separator 23 in parallel with the discharges 22 from compressors I0 and I1 and the compressed refrigerant output of air conditioning compressor 15 is condensed and conducted to receiver 28 in the aforedescribed manner. Liquid refrigerant from receiver 28 is supplied through conduit 30 to the air-conditioning evaporator 31 having the usual controls {not shown) and conduit 32 connects the evaporator I to the suction side of air-conditioning compressor 15 to complete the cycle.

Another portion of the overall system illustrate in FIG, I that forms no part of the invention of this application is the discharge accumulator and the oil separator 23 and its associated oil return means although it will be understood that some form of oil separator and return must be provided in a refrigeration system of this type. The arrangement illustrated in FIG. 1 is of the type more fully disclosed in my copending application Ser. No. 8l5,452 entitled Refrigeration System Oil Separator" but will be briefly described here for completeness. The vessel comprising the discharge accumulator and oil separator 23 serves to separate the oil from gaseous refrigerant discharged from the compressors and to maintain an oil supply 33 in the bottom of the vessel. An oil line 34 is connected through a header to individual conduits returning to the crankcases of each of the compressors 1045. Each such oil return conduit has a separate valve 35 the operation of which is individually controlled by a floatactuated sensor 36 associated with the compressor to which that oil return line and valve are connected. Thus as the sensor 36 associated with a compressor senses a drop in the desired oil level in that compressor the associated valve 35 is opened to return oil to that compressor whereby the desired oil levels in each of the compressors is independently maintained.

Returning to the refrigeration system of FIG. I, the receiver 28 is connected through a conduit 40 through a pressure regulating valve 41 to a liquid refrigerant header 42. The pressure regulating valve 41 functions to maintain a reduced and constant pressure in liquid header 42 and valve 41 may be of the type commonly known in the industry as a "crankcase pres sure regulator" or CPR valve. The predetermined pressure maintained in liquid header 42 by valve 41 is selected to be lower than the lowest condensing or head pressure likely to be encountered even under the most extreme conditions and higher than the highest evaporator suction pressure such as produced by standard temperature compressors l0 and II. The liquid header 42 is connected through conduits 43 to the evaporators 44 of the refrigerated fixtures. For simplicity of illustration only one evaporator 44 and six branch conduits 43 are shown although one or more evaporators will be associated with each conduit 43 and in most commercial instal lations of this type there will be a great many more than six branch conduits 43.

As is conventional each evaporator 44 is provided with an expansion valve 45 and for purposes of bypassing expansion valve 45 during hot gas defrosting a bypass line 46 having a check valve is provided. in addition each evaporator will be provided with a temperature responsive control such as a conventional liquid line solenoid or, as shown in the drawing. a suction pressure regulator valve 47 Each evaporator 44 is connected through a suction conduit 48 to a three way valve 49 adapted to selectively connect the suction line 48 to either a common hot gas header 511' or one of the three suction headers 51, 52 or 53. [t is to be noted that as viewed in FIG. 1 the two right hand three-way valves 49 are connected to standard temperature suction header 51 whereby the associated fixtures would be standard temperature fixtures, the two mid dle three-way valves 49 are connected to low temperature suction header 52 whereby the associated fixtures would be low temperature fixtures. and the two left hand three-way valves 49 are connected to ultra-low temperature suction header 53 whereby the fixtures associated with those two valves would be ultralow temperature fixtures. Standard temperature suction header 51 is connected through conduit 54 to the suction side 21 of compressors and 11. Low temperature suction header 52 is connected through conduit 55 to the suction side 18 of low temperature compressors 12 and 13. Ultralow temperature suction header 53 is connected through conduit 56 to the suction side of compressor 14. During normal refrigerating operation of a given evaporator 44 the associated three way valve 49 is positioned to connect the suction conduit 48 to the associated suction header (S1, 52 or 53) to return the evaporated refrigerant to the appropriate compressor whereby the appropriate suction pressure is maintained on that evaporator and yet the liquid refrigerated is supplied to all ofthe cvaporators from the common liquid heater 42.

A bypass line 57 with a check valve 58 is connected from the ultra-low temperature suction line 56 to the discharge side 16 of the ultralow temperature compressor 14 for allowing the bypass of refrigerant upon the failure of compressor 14 or the temporary return of an excessive quantity of refrigerant to compressor 14. in essence the suction from the ultralow temperature fixtures will then be connected to the suction side 18 of the low temperature compressors l2 and 13 until the malfunction of compressor 14 is corrected or the excess return is reduced,

Referring again to the operation of pressure regulating valve 41 in supplying liquid refrigerant to the header 42. it is well known in the industry that a pressure reduction of this type in the liquid refrigerant will cause some of the liquid refrigerant to evaporate, commonly called flash gas, to cool the liquid to the saturation temperature for that pressure unless the refrigerant is subcooled to such temperature before the pressure reduction. A common example of the creation of flash gas in every refrigeration system occurs at the expansion valve since the pressure is reduced from the liquid line pressure to the evaporator pressure and the flash gas serves to lower the temperature of the remaining liquid from the liquid line temperature to the evaporator temperature The quantity of flash gas created will depend on this temperature differential but it is to noted that such flash gas produces no useful work in the evaporator in cooling the refrigerated fixture although the flash gas obviously represents part ofthe load on the compressor since it must be returned and recompressed. Of course the flash gas also represents part of the volume that must be conducted by the suction conduits and in its liquid form, part of the liquid that must be conducted by the liquid supply lines whereby such piping must be sized to accommodate those volumes. However any portion of the flash gas usually produced at the expansion valve that can be eliminated upstream thereof will allow a corresponding reduction in the necessary size of the liquid supply lines and suction lines and, as hereinafter described such flash gas elimination is accomplished by the system ofFlG. 5.

Immediately downstream of the pressure regulating valve 41 is a separating vessel or chamber 60 connected to the liquid header 42 in a manner for causing the flash gas produced by the pressure reduction to enter the chamber 60. The chamber 60 contains both gaseous refrigerant and liquid refrigerant as shown. The flash gas collected in chamber is returned through a conduit 61 connected from the top of the chamber to conduit 54 returning to the suction side 21 of standard ternperature compressors 10 and 11. Means are provided for con trolling the removal of flash gas through conduit 61 to avoid either the excess accumulation of flash gas in chamber 60 or the inadvertent return of liquid refrigerant to the compressors and, as shown in the drawings. these means may include a float control 62 provided in chamber 60 and connected to and controlling the operation of a valve 63 in conduit 61. The controlled operation of valve 63 by float control 62 may be either an on-off switching between high and low levels of liquid in the chamber 60 or a modulating control directly related to the level of liquid in chamber 61) 1t is to be noted that float con' trol 62 may also serve as an indicator or alarm for the occurrence ofa below normal supply ofliquid refrigerant as for example due to the loss of refrigerant through a leak since chamber 60 will always be partially tilled with liquid refrigerant unless there simply is no additional liquid refrigerant available from receiver 28 through conduit 40 and valve 414 Thus it may be seen that the pressure and temperature reduction of the liquid refrigerant by the regulating valve 41 from the pressure and temperature in receiver 28 to the desired lower level in header 42 is accomplished automatically and continually without employing a subcooler or the like and yet the resultant flash gas is immediately removed and returned directly to the compressors in order to accomplish hot gas defrosting of the evaporators in the system of FIG. 1 as thus far described a variety of arrangements may be used but 1 have illustrated a still further improvement in my invention of a refrigeration system with hot gas defrosting and this is my preferred arrangement. When it is desired to defrost a given evaporator 44 the associated three-way valve 49 is repositioned from its normal rel'rigcrat ing connection wherein suction conduit 48 is connected to the suction header (51, S2 or 53) to the position wherein suction conduit 48 is connected to hot gas header 50. Simultaneously a solenoid valve 64 in conduit 20 is closed thereby eliminating the connection of the discharge sides 19 of low-temperaturc compressors 12 and 13 to the suction side 2] of standard temperature compressors 10 and II. A conduit 65 connects the discharge sides 19 of low temperature compressors 12 and 13 to hot gas header 50 to supply the hot gaseous refrigerant for defrosting from compressors l2 and 13. A pressure regulating valve 66 is provided in conduit 65 and functions to limit the pressure in conduit 65 so it will not exceed a predetermined constant downstream pressure of a relatively high level related to the temperature of gas desired for accomplishing the defrosting. ln order to avoid excessive pressures on the discharge sides of compressors l2 and 13 as might be produced by the pressure regulating of valve 66, a bypass con duit 67 is connected from the compressor discharge sides 19 to the accumulator and oil separator vessel 23. A pressure regulating valve 68 of the type for regulating upstream pressure if provided in bypass conduit 67 and is set for a slightly higher pressure than the setting of valve 66 whereby any excess refrigerant not being passed through valve 66 for hot gas defrosting purposes will be discharged through bypass conduit 67 into the vessel 23 there by entering the normal refrigeration cycle A check valve 69 is provided in bypass conduit 67 to prevent reverse flow therethrough such as during normal refrigeration operation of the system (when there is no hot gas defrosting being performed) whereby valve 64 would be open and the pressure on the discharge sides 19 of compressors 12 and 13 would be substantially lower than the pressure in vessel 23 A dcsuperheater 70 is provided in association with conduit 65 between pressure regulating valve 66 and the hot gas header 50 for eliminating at least most of the superheat in the gaseous refrigerant whereby the refrigerant will be more nearly its saturation temperature for that pressure thereby avoiding excessive heating of the conduits leading to the evaporators. Desuperheater 7G is shown as a writer-cooled type with the water flow therethrough controlled by valve 71 which is in turn controlled by a temperature responsive sensor 72 associated with the conduit 65 downstream of the desuperheater 70. In this manner the temperature of the hot gas may be lowered to approximately the saturation temperature for the pressure setting of regulator valve 66. It will be obvious to those skilled in the art that any form of desuperheater may be used for accomplishing this objective.

The desuperheated gaseous refrigerant passed from hot gas header 50 through the three-way valve or valves 49 associated with the evaporators to be defrosted, then passes in a reverse direction through the suction conduit 48 to the evaporator 44, and thence through the bypass conduit 46 into the liquid line 43. A check valve 73 is provided in each liquid line 43 to prevent the flow of the defrosting refrigerant into the liquid header 42. A branch conduit 74 connects each liquid line 43 to a condensate header 75 for passing the defrosting refrigerant from line 43 to header 75. Each conduit 74 is provided with a check valve 76 to preclude the reverse flow of refrigerant from header 75 into the liquid line 43 of an evaporator operating in its normal refrigerating cycle which would otherwise occur due to the pressure differentials as will hereinafter appear more fully. A conduit 77 having a pressure regulating valve 78 connects the condensate header 75 to the liquid header 42 immediately downstream of the pressure regulating valve 41 for returning the refrigerant used in defrosting to the refrigerating cycle. Valve 78 functions to maintain a constant upstream pressure (i.e., in header 75) and such pressure is substantially higher than the pressure in liquid header 42 as regulated by valve 41. This higher pressure in condensate header 75 precludes the flow of liquid refrigerant into the header 75 from a liquid line 43 during normal refrigerating operation.

it is quite common for significant quantities of gaseous refrigerant to be returned with the defrosting refrigerant after passing through the evaporator being defrosted, particularly in the final stages of defrosting. In various previously conven tional systems the reintroduction of this gaseous refrigerant into the liquid refrigerant supply caused substantial malfunctioning of those evaporators operating on their normal refrigerating cycles. Moreover in the present system as the refrigerant passes through regulating valve 78 there is a pressure reduction causing flash gas to pass into header 42 along with the gaseous refrigerant returned from defrosting. However chamber 60 is positioned downstream from the connec tion of conduit 77 to header 42 whereby such gaseous refrigerant will enter chamber 60 along with the flash gas produced by the pressure reduction by valve 41 and all such gas is separated in chamber 60 and removed through conduit 61 as previously described. Thus the refrigerant actually supplied through lines 43 to evaporators for refrigerating function is essentially free of gaseous refrigerant and is at a reduced temperature and pressure. It is to be noted that in FIG. I, of the six separate evaporator systems illustrated. the valves 49 of three of them are positioned for hot gas defrosting of the as sociated evaporator and, numbering from the left, these are the second, third and sixth valves.

In the installation and operation of a refrigeration system as shown in FIG. I a variety of different pressure settings may be selected for the various regulating valves 41, 66, 68 and 78. These settings will depend on a number of factors such as the particular refrigerant selected and the common ambient conditions encountered. For purposes of illustration and without limiting the scope of this invention, the system of FIG. 1 will now be described in connection with typical pressure settings that might be selected when using refrigerant 502. The pressure regulating valve 41 may be set for 100 p.s.i. with a resultant temperature of the liquid refrigerant in liquid header 42 of approximately 51 F. A desirable setting of hot gas defrosting pressure regulating valve 66 might be 2l5 p.s.i. whereby the defrosting gaseous refrigerant, after desuperheat ing by desuperheater 70, would be slightly over I F., perhaps i it) F. to avoid any actual condensing by desuperheater 70. In turn regulating valve 68 would be set a few pounds higher than valve 66 perhaps 2 I 8 p.s.i. Finally regulating valve 78 might be set to maintain I p.s.i. pressure in condensate header 75 whereby a pressure differential of about 30 p.s.i. is created between hot gas header 50 and condensate header 75 to cause the flow of defrosting refrigerant through conduit 48 and evaporator 44, and to be returned through line 43. This predetermined pressure differential results in predictable and consistent defrosting refrigerant flow through the evaporators whereby an appropriate defrosting time may be selected for each evaporator and a consistent defrosting under all ambient conditions will result. In a supermarket installation of this type, the typical suction pressures on the three levels of com pressors for this arrangement might be as follows; 4] p.s.i. on standard temperature compressors l0 and 11, I2 p.s.i. on lowtemperature compressors l2 and 13, and 0 p.s.i. (gauge) on ultralow temperature compressor I4. Air-conditioning compressor 15 would normally be set for a suction pressure of around 80 p.s.i. Thus it may be seen that the pressure differential across the expansion valve 45 of a given evaporator 44 will be substantially constant under all operating and ambient conditions which pressure differential will be approximately 59 p.s.i. for standard temperature evaporators, 88 p.s.i. for lowtemperature evaporators, and I00 p.s.i. for the ultralow temperature evaporators. As will readily appear to those skilled in the art, this will greatly improve the ability to select and properly set the expansion valves and will improve their operation.

In a central system of the type shown in FIG. I and described wherein the compressors are centrally located and yet the evaporators are remotely located, the various headers that have been described will also be centrally located with the compressors. Thus for example the liquid header 42 and flash gas separating chamber 60 will be closely associated with the compressors whereby the removal and return of the flash gas to the compressor suctions is simple and efficient. In contrast, the liquid lines 43 and suction liens 48 will be extremely long for connecting the refrigerated fixtures to the centrally located compressors and headers. Thus due to the aforedescribed reduction in line sizes resulting from climina tion of a substantial portion of the flash gas that is normally created at the expansion valve 45, there is a substantial savings in the cost of the pipes 43 and 48 not heretofore possible. Moreover the recompression of this undesirable flash gas is accomplished by the standard temperature compressor thereby permitting a reduction in the size of the low and ultralow temperature compressors. By providing a constant and relatively low liquid header pressure through the use of regulating valve 41 it is possible to permit the condensing or head pressure to vary over a wide range as dictated by ambient conditions including permitting the pressure to drop to otherwise unacceptably low pressures which produces substantial reduction in the power consumption of the compressors.

Referring now to FIG. 2, a portion of a refrigeration system is shown that may be substituted for a corresponding portion of the system more fully disclosed in FIG. I and to the extend that components of the system of FIG. 2 correspond to or are substitutes for the system of FIG. I like numerals in the I00 series will be employed. The compressor has its suction side 121 connected through conduit 154 to the suction header of the evaporators operated by compressor 110. Also it may be seen that if the system of FIG. 2 were a compound compressor arrangement as is shown in FIG. I the suction side 121 and conduit 154 may also be connected to the discharge side of another compressor or compressors such as the low temperature compressors l2 and I3 shown in FIG. I. The corn pressor Ill] discharges refrigerant through a conduit 127 to a high side float device I80. A conduit 127A is connected from the high side float device to a receiver I28. The high side float device 180 is a relatively conventional apparatus for allowing only the passage of liquid refrigeration therethrough from its inlet 18! to its outlet 182 and this has been diagrammatically shown by the float arrangement adapted to close the IOIOZS 0055 outlet 182 upon the lowering of the liquid level in device 180. In normal operation the pressure drop between conduit 127 and receiver 128 occurs across device 180.

As shown in FIGS. 2 and 3 the receiver 128 is of a unique design which is adapted to enhance the separation of gaseous and liquid refrigerant and yet retain many of the known advantages of a surge type receiver. Receiver 128 includes a cylindrical vessel 183 positioned with its axis horizontal and having a conduit 184 passing horizontally therethrough along the lower portion of the vessel. Conduit 184 is provided with perforations or slots 185 along its length and around its circumference. At least some of the slots 185 are provided along the uppermost portion of conduit 184 to allow flash gas to escape from the conduit as shown by arrow 186. Additional slots 185 are positioned in the lower portion of conduit 184 to allow liquid refrigerant to enter the conduit as shown by arrows 187. The inlet end of conduit 184 is connected to conduit 127A and the outlet end is connected to the main liquid header 142. The conduit 161 is connected from the upper portion of receiver 128 to the suction side 121 of the compressor for removing flash gas from the receiver 128. A pressure regulating valve 188 is provided in conduit 161 and serves to maintain a predetermined constant pressure in receiver 128 by controlling the rate of flash gas flow through the valve. By way of comparison with the aforedescribed system of FIG. 1, valve 188 is set at 100 p.s.i. and will open at any pressure above 100 p.s.i. for passing the flash gas to the compressor thereby maintaining the supply of liquid refrigerant through header 142 at 100 p.s.i., comparable to the manner in which valve 41 of FIG. 1 maintains the liquid line pressure of 101] p.s.i. Thus it will be seen that the combination of high side float device 180 and pressure regulating valve 188, commonly known as an EPR valve, serve to reduce the pressure from the head pressure in condenser 126 to the constant low liquid line pressure of 100 p.s.i. in header 142. The flash gas produced in such pressure reduction is separated in receiver 128 and removed through conduit 161. Conduit 177 is connected to conduit 127A between the high side float device 180 and receiver 128 to return the refrigerant used in defrosting the evaporators to this point in the systems, similar to the function of aforedescribed conduit 77. A pressure regulating valve 178 maintains a constant upstream pressure in conduit 177 which, in comparison with the aforedescribed system of FIG. 1, would be 185 p.s.i. It may be seen that the flash gas created by the pressure reduction through valve 178 plus the gaseous refrigerant being returned from the fixtures being defrosted is carried into receiver 128, is separated therein, and then removed through conduit 161. Thus the receiver 128 also serves as a gas separating chamber comparable to aforedescribed chamber 60. It is to be noted that if an air conditioning system is in cluded in the system of FIG. 2 the liquid refrigerant will be supplied through a conduit 130 from the vessel of the high side float device 180 whereby the liquid pressure will be sufficiently high to supply the air conditioning evaporator.

Among the advantageous difference between the system of FIG. 2 and the previously described system of FIG. 1 obviously is the elimination of one vessel, that is, the receiver 128 of FIG 2 replaces both the receiver 28 and the separating chamber 60 of the FIG. 1 system. Moreover the flash gas removal is greatly simplified in that the regulating valve 188 supplants the float control 62 and valve 63 in the system of FIG. In turn this flash gas removal arrangement of FIG. 2 is more dependable and less likely to allow any liquid refrigerant to flow back to the compressor suction in that the receiver 128 is normally of a sufficient size to contain the entire refrigerant charge of the system therefore liquid refrigerant would not reach the outlet to conduit 161. Another advantageous result in the system of FIG. 2 is the creation of an automatic condenser flooding control since creation of an automatic condenser flooding control since receiver 128 is a modified surge type receiver whereby flooding will occur whenever the condenser prcssure drops below the pressure setting of valve 188, 100 p.s.i. in our example.

Referring now to FIG. 4, another modified form of the system of this invention is illustrated in greatly simplified form. Again insofar as the components of this system correspond to components of the systems of FIGS. 1 and 2 numerals in the 200 series will be employed. Compressor 210 has its discharge side 222 connected to the condenser 226 which in turn is connected through conduit 227 to the receiver 228 which is illustrated as a surge-type receiver. A liquid refrigerant line 240 has a solenoid valve 289 therein which is in a normally open position during refrigerating operation of the system. The liquid line 240 is connected to a liquid header 242 of enlarged diameter with the liquid line 240 actually ex tending within a substantial portion 0 the length of the header 242. Within header 242 line 240 is provided with apertures 290 whereby the refrigerant passing therethrough into header 242 is separated into gaseous and liquid refrigerant without creating excessive turbulence in header 242. Thus header 242 in combination with the apertured portion of line 240 comprise a separating vessel or chamber similar to chamber 60 in the system of FIG. 1. Liquid refrigerant lines 243 connect the bottom of header 242 to the evaporators 244 through the expansion valves 245 in the manner heretofore described. Suction lines 248 are connected to three-way valves 249 for selectively connecting the suction lines to either the suction header 251 or the hot gas header 250. The hot gas header 250 is connected through a conduit 265 to the top of receiver 228 for supplying hot gaseous refrigerant for defrosting which gaseous refrigerant will be at the condensing or head pressure and at substantially saturation temperature. As shown in FIG. 4 the right-hand three-way valve 249 is positioned for supplying hot gas to the right-hand evaporator 244 for defrosting and the resultant defrosting refrigerant returns through line 243 to the liquid header 242 where any resultant gaseous refrigerant will rise to the top of the header. In order to provide a pressure differential incentive for the return of defrosting refrigerant to header 242 the solenoid valve 289 in liquid refrigerant supply line 240 is closed whereby the liquid refrigerant must flow through a bypass line 291. A pressure reducing valve 292 is positioned in line 29] for producing a predetermined pressure differential thereacross. as for example about 30 p.s.i. Thus during defrosting the pressure in liquid header 242 will be around 30 p.s.i. less than the pressure in receiver 228 which constitutes the pressure of the hot gaseous defrosting refrigerant. Any flash gas created by the pressure drop across valve 292 will be separated in header 242. A float switch 262 is positioned in one end of header 242 for sensing the liquid level in the header and controls a valve 263 positioned in a suction line 261 connected to the upper portion of header 242. Suction line 261 is connected through an accumulator vessel 293 to the suction side 221 of compressor 210 to avoid passing any liquid refrigerant from line 261 directly to the intake if the compressor. The float switch 262 opens and closes valve 263 in response to the liquid level in header 242 to maintain an adequate quantity of liquid refrigerant therein for supplying the evaporators and yet precluding liquid refrigerant from flowing back through suction line 26]. Float switch 262 and valve 263 may be of either an on-off type or modulating between open and closed positions for controlling the liquid level in the header. An adjustable throttling valve 294 may be provided in suction line 261 for further control of the flow therethrough.

Referring to FIG. 5, another modified form of the system of this invention if illustrated in simplified form similar to the illustration of FIG. 4. Again simiiar components in this system of FIG. 5 will be given numerals in the 300 series corresponding to the previous numerals. The compressor 310 has its discharge 322 connected to condenser 326 and thence through conduit 327 to receiver 328. Liquid line 340 is connected to liquid header 342 which in turn is connected through liquid lines 343 to the evaporators 344. A suction line 348 is connected from each evaporator 344 through a threeway valve 349 to both the suction header 351 and hot gas header 350. In this system the hot gas header is illustrated as being connected directly to the compressor discharge 322. When defrosting of an evaporator is desired the three-way valve 349 is repositioned to connect the hot gas header 350 to the suction line 348 as shown by the right-hand valve 349. Simultaneously a solenoid valve 395 in liquid supply line 343 from liquid header 342 is closed and a second solenoid valve 396 in branch conduit 374 is opened for conducting the defrosting refrigerant to condensate header 375. Under nor' mal refrigerating operation of an evaporator 344 the valve 395 is open and valve 396 is closed. The defrosting refrigerant passes from condensate header 375 through conduit 377 and check valve 397 to a separating vessel or chamber 360 where the liquid and gaseous phases of the refrigerant are separated. The top portion of chamber 360 is connected through a suction line 361 to the suction side 321 of the compressor 310 and a pressure reducing valve 392 is positioned in conduit 361 for controlling the flow therethrough. Valve 392 is set to maintain a predetermined pressure differential between the discharge side 322 of the compressor and the condensate header 375 such as around a 30 psi. differential, whereby an incentive is produced to cause the flow of the defrosting refrigerant through the evaporator to the condensate header. Means are provided for sensing and controlling the liquid level in chamber 360 between maximum and minimum limits and as shown in the drawings these means may include upper and lower float switches 398 and 399. During the defrosting of one or more evaporators, as the liquid level in chamber 360 rises to the maximum desired level as sensed by float switch 398 a solenoid valve 400 in suction line 361 is closed, a solenoid valve 401 is opened in a high pressure line 401 between the compressor discharge side 322 and chamber 360, and a third solenoid valve 403 positioned in liquid supply line 340 is closed. A drain line 404 is connected from chamber 360 to liquid line 340 downstream from the location of valve 403 whereby the increased pressure in chamber 360 through line 402 will cause the accumulated liquid refrigerant in the chamber 360 to become the normal liquid refrigerant supply in line 340. When the predetermined minimum level of liquid refrigerant in chamber 360 is reached as sensed by float switch 399, valves 400 and 403 are returned to their open positions and valve 401 is closed. A check valve 405 in drain line 404 prevents the flow ofliquid refrigerant from liquid line 340 into chamber 360. Thus a predetermined pressure differential is again provided as incentive for the flow of hot gas defrosting refrigerant through the evaporators and of the returning defrosting refrigerant to chamber 360 where the gaseous phase is removed and returned to the compressor suction while only the liquid phase is returned to the liquid refrigerant supply for the evaporators.

Having fully described my invention in connection with selected embodiments and arrangements by way of example, it is to be understood that my invention is not limited to such specific embodiments and arrangements as disclosed but rather is of the full scope of the appended claims.

I claim:

I. A refrigeration system having multiple refrigerated fixtures with hot gas type defrosting of such fixtures, comprising, compressor means for compressing refrigerant and having low pressure suction and high pressure discharge sides, condenserreceiver means for condensing the compressed refrigerant and supplying liquid refrigerant, a plurality of evaporators associated with the refrigerated fixtures and having means normally connecting said evaporators in parallel between said condcnserreceiver means and said suction side of the compressor means with means for evaporating the refrigerant and cooling said fixtures, a chamber means for separating liquid and gaseous refrigerant. valve and conduit means connecting at least one of said evaporators between the high pressure side of the system and said chamber means and operable for selectively supplying hot gaseous refrigerant to said evaporator for defrosting and for conducting the resultant liquid and gaseous refrigerant directly to said chamber means for separating the phases, said chamber means being in communication with said means normally connecting said condensenreceiver means and said evaporators for returning the resultant liquid refrigerant phase produced by defrosting from said chamber means into the liquid refrigerant supply for said evaporators, and means restrictively connecting said chamber means to the suction side of said compressor means for controllably returning the separated gaseous refrigerant from said chamber means to said compressor means.

2. The system of claim I wherein said means restrictively connecting said chamber means and compressor suction side includes valve means operably responsive to the presence of excess gaseous refrigerant in said chamber for controlling the communication with said compressor suction side.

3. The system of claim 2 wherein said valve means has means for controlling the pressure in said chamber means whereby the excess gaseous refrigerant is controllably removed.

4. The system of claim 1 wherein a float control means is positioned in said chamber and responsive to the liquid level therein and said means restrictively connecting said chamber means and compressor suction side includes a valve means controlled by said float control means,

5. The system of claim 1 wherein a single vessel includes both said chamber means and the refrigerant retaining receiver portion of the condenser-receiver of the system.

6. The system of claim 5 wherein said vessel is comprised of an elongated cylinder positioned horizontally, and a perforated conduit extends along the bottom of the cylinder with such perforations admitting liquid refrigerant and allowing exit of gaseous refrigerant into the vessel, said conduit having ends comprising the inlet and outlet of the refrigerant receiver formed by the vessel.

7. The system of claim 1 wherein means are provided for reducing the pressure of the liquid refrigerant supply before conducting such refrigerant to the evaporators, said chamber means being in direct communication with such reduced pressure liquid refrigerant supply between said pressure reducing means and the evaporators for receiving and separating flash gas formed in such pressure reduction, said flash gas also being removed from said chamber means by said restrictive connecting means to said compressor suction side.

8. The system of claim 7 wherein pressure reducing means produces a predetermined low pressure having a saturation temperature below normal room temperature in said liquid refrigerant supply and a predetermined pressure differential across said evaporators unrelated to the system head pressure.

9 The system of claim 7 wherein said compressor means, condenser-receiver means and chamber means are all cen trally located together and said evaporators and fixtures are remotely located with connecting conduits between such locations, said conduits being of reduced size comparable to the reduced refrigerant conducted by reason of eliminating said flash gas.

10. The system of claim 1 wherein said valve and conduit means includes two pressure regulating valve means operable for controlling maximum and minimum pressures for supplying said hot gaseous refrigerant for defrosting within a small predetermined pressure range under all ambient and operating conditions for controlled defrosting.

II. The system of claim 10 wherein said valve and conduit means includes another pressure regulating valve means for producing a predetermined back pressure on the refrigerant returning from defrosting.

[2. The system of claim 11 wherein said other pressure regulating valve means causes a pressure reduction in the defrosting refrigerant before entering said chamber means and any resultant flash gas is separated in said chamber means.

13. The system of claim 1 wherein means are provided in said chamber means for sensing maximum and minimum desired liquid levels in said chamber means, valve controlled means connecting said chamber means to the compressor means discharge side, valve means in said means normally connecting said condenser-receiver to said evaporators upstream of the communication with said chamber means, and said sensing means operating said valve controlled means and said valve means to periodically close said valve means and open said valve controlled means upon sensing said maximum level to supply the liquid refrigerant for the evaporators from said chamber means and to reopen said valve means and close said valve controlled means upon sensing said minimum level.

14. The system of claim 13 wherein said restrictive connecting means includes a valve and pressure reducing valve means normally maintaining a pressure differential between said chamber means and normal liquid refrigerant supply, said chamber means having check valve means preventing reverse flow of refrigerant from the liquid refrigerant supply means. and said sensing means also operates said valve in said restric tive connecting means to the same positions as said valve means in said liquid supply.

15. A refrigeration system having multiple refrigerated fixtures with hot gas type defrosting of such fixtures, comprising, compressor means for compressing refrigerant and having low pressure suction and high pressure discharge sides, condenserreceiver means for condensing the compressed refrigerant and supplying liquid refrigerant, a plurality of evaporators associated with the refrigerated fixtures and having means normally connecting said evaporators in parallel between said condenser-receiver means and said suction side of the compressor means with means for evaporating the refrigerant and cooling said fixtures, a chamber means for separating liquid and gaseous refrigerant, valve and conduit means connecting at least one of said evaporators to the high pressure side of the system to supply hot gaseous refrigerant to said evaporator for defrosting and for conducting the resultant refrigerant condensate to said chamber means, said chamber means being in communication with said means normally connecting said condenserreceiver means to said evaporators for returning liquid condensate refrigerant from defrosting into the liquid refrigerant supply for said evaporators, means restrictively connecting said chamber means to the suction side of said compressor for controllably returning gaseous refrigerant from said chamber means to said compressor means, and valve means for affirmatively producing a reduced pressure in said chamber means relative to the pressure of said hot gase ous defrosting refrigerant for producing a pressure differential to cause the defrosting refrigerant to flow through the evaporators into said chamber means.

16, The refrigeration system of claim 15, wherein said valve means produces a predetermined and relatively low pressure in the liquid refrigerant supply.

17, The refrigeration system ofclaim 15, wherein said valve means produces a predetermined pressure drop in said liquid refrigerant supply between said condenser-receiver and the location of communication of said chamber means.

18. The refrigeration system of claim 15, wherein said valve means produces a predetermined pressure differential between said compressor discharge side and said chamber means and the absolute pressures vary with discharge side pressure variations.

19. The refrigeration system of claim 15, wherein said valve means also affirmatively produces a like reduced pressure in said liquid refrigerant supply upstream of the location of said chamber means, and flash gas created by said liquid refrigerant pressure reduction is separated in said chamber means and also removed by said restrictive connecting means in a controlled manner to said compressor.

20. In a closed cycle refrigeration system employing a compressor, condenser and receiver for supplying liquid refrigerant and multiple evaporators for multiple refrigerated fixtures wherein hot gas defrosting of such fixtures is provided, the improvement comprising, a vessel having chamber means for separating liquid and gaseous refrigerant, valve and conduit means connecting said evaporators to said vessel for selectively connecting it at least one of sad evaporators between the high pressure side of the system and said vessel to supply hot gaseous refrigerant to said evaporator for defrost ing and for conducting the resultant liquid and gaseous refrigerant directly to said chamber means for separating the phases, said vessel having means in communication with the normal supply of liquid refrigerant to said evaporators for returning the liquid refrigerant resulting from defrosting into the liquid refrigerant supply for said evaporators, and means restrictively connecting said chamber means to the suction side of said compressor for controllably returning gaseous refrigerant from said chamber means to said compressor means.

21. The refrigeration system improvement of claim 20 wherein a regulating valve means is provided in communication with said chamber means and affirmatively produces a pressure differential between said chamber means and the supply of hot gaseous defrosting refrigerant to cause the defrosting refrigerant to flow through the evaporators to said chamber means.

22v The refrigeration system improvement of claim 20 wherein float control means are provided in said chamber means for maintaining a desired liquid level in said chamber means.

23. The refrigeration system improvement of claim 20 wherein pressure regulating valve means are provided for controlling to three predetermined pressure levels the liquid refrigerant supply, the hot gaseous defrosting refrigerant supply and the resultant refrigerant condensate returning from defrosting,

24. The refrigeration system improvement of claim 21 wherein valve and conduit means also serve to activate said regulating valve means only during the defrosting of an evaporator.

25. The refrigeration system improvement of claim 20 wherein pressure regulating valve means are provided for producing a predetermined and constant low pressure in both the normal supply of liquid refrigerant and said vessel, and the refrigerant at said constant low pressure has a saturation temperature below the normal room temperature.

26. The refrigeration system improvement of claim 20 wherein two pressure regulating valve means are provided for maintaining the pressure of said hot gaseous refrigerant within a small predetermined range under all ambient and operating conditions. 

1. A refrigeration system having multiple refrigerated fixtures with hot gas type defrosting of such fixtures, comprising, compressor means for compressing refrigerant and having low pressure suction and high pressure discharge sides, condenserreceiver means for condensing the compressed refrigerant and supplying liquid refrigerant, a plurality of evaporators associated with the refrigerated fixtures and having means normally connecting said evaporators in parallel between said condenser-receiver means and said suction side of the compressor means with means for evaporating the refrigerant and cooling said fixtures, a chamber means for separating liquid and gaseous refrigerant, valve and conduit means connecting at least one of said evaporators between the high pressure side of the system and said chamber means and operable for selectively supplying hot gaseous refrigerant to said evaporator for defrosting and for conducting the resultant liquid and gaseous refrigerant directly to said chamber means for separating the phases, said chamber means being in communication with said means normally connecting said condenser-receiver means and said evaporators for returning the resultant liquid refrigerant phase produced by defrosting from said chamber means into the liquid refrigerant supply for said evaporators, and means restrictively connecting said chamber means to the suction side of said compressor means for controllably returning the separated gaseous refrigerant from said chamber means to said compressor means.
 2. The system of claim 1 wherein said means restrictively connecting said chamber means and compressor suction side includes valve means operably responsive to the presence of excess gaseous refrigerant in said chamber for controlling the communication with said compressor suction side.
 3. The system of claim 2 wherein said valve means has means for controlling the pressure in said chamber means whereby the excess gaseous refrigerant is controllably removed.
 4. The system of claim 1 wherein a float control means is positioned in said chamber and responsive to the liquid level therein and said means restrictively connecting said chamber means and compressor suction side includes a valve means controlled by said float control means.
 5. The system of claim 1 wherein a single vessel includes both said chamber means and the refrigerant retaining receiver portion of the condenser-receiver of the system.
 6. The system of claim 5 wherein said vessel is comprised of an elongated cylinder positioned horizontally, and a perforated conduit extends along the bottom of the cylinder with such perforations admitting liquid refrigerant and allowing exit of gaseous refrigerant into the vessel, said conduit having ends comprising the inlet and outlet of the refrigerant receiver formed by the vessel.
 7. The system of claim 1 wherein means are provided for reducing the pressure of the liquid refrigerant supply before conducting such refrigerant to the evaporators, said chamber means being in direct communication with such reduced pressure liquid refrigerant supply between said pressure reducing means and the evaporators for receiving and separating flash gas formed in such pressure reduction, said flash gas also being removed from said chamber means by said restrictive connecting means to said compressor suction side.
 8. The system of claim 7 wherein pressure reducing means produces a predetermined low pressure having a saturation temperature below normal room temperature in said liquid refrigerant supply and a predetermined pressure differential across said evaporators unrelated to the system head pressure.
 9. The sYstem of claim 7 wherein said compressor means, condenser-receiver means and chamber means are all centrally located together and said evaporators and fixtures are remotely located with connecting conduits between such locations, said conduits being of reduced size comparable to the reduced refrigerant conducted by reason of eliminating said flash gas.
 10. The system of claim 1 wherein said valve and conduit means includes two pressure regulating valve means operable for controlling maximum and minimum pressures for supplying said hot gaseous refrigerant for defrosting within a small predetermined pressure range under all ambient and operating conditions for controlled defrosting.
 11. The system of claim 10 wherein said valve and conduit means includes another pressure regulating valve means for producing a predetermined back pressure on the refrigerant returning from defrosting.
 12. The system of claim 11 wherein said other pressure regulating valve means causes a pressure reduction in the defrosting refrigerant before entering said chamber means and any resultant flash gas is separated in said chamber means.
 13. The system of claim 1 wherein means are provided in said chamber means for sensing maximum and minimum desired liquid levels in said chamber means, valve controlled means connecting said chamber means to the compressor means discharge side, valve means in said means normally connecting said condenser-receiver to said evaporators upstream of the communication with said chamber means, and said sensing means operating said valve controlled means and said valve means to periodically close said valve means and open said valve controlled means upon sensing said maximum level to supply the liquid refrigerant for the evaporators from said chamber means and to reopen said valve means and close said valve controlled means upon sensing said minimum level.
 14. The system of claim 13 wherein said restrictive connecting means includes a valve and pressure reducing valve means normally maintaining a pressure differential between said chamber means and normal liquid refrigerant supply, said chamber means having check valve means preventing reverse flow of refrigerant from the liquid refrigerant supply means, and said sensing means also operates said valve in said restrictive connecting means to the same positions as said valve means in said liquid supply.
 15. A refrigeration system having multiple refrigerated fixtures with hot gas type defrosting of such fixtures, comprising, compressor means for compressing refrigerant and having low pressure suction and high pressure discharge sides, condenser-receiver means for condensing the compressed refrigerant and supplying liquid refrigerant, a plurality of evaporators associated with the refrigerated fixtures and having means normally connecting said evaporators in parallel between said condenser-receiver means and said suction side of the compressor means with means for evaporating the refrigerant and cooling said fixtures, a chamber means for separating liquid and gaseous refrigerant, valve and conduit means connecting at least one of said evaporators to the high pressure side of the system to supply hot gaseous refrigerant to said evaporator for defrosting and for conducting the resultant refrigerant condensate to said chamber means, said chamber means being in communication with said means normally connecting said condenser-receiver means to said evaporators for returning liquid condensate refrigerant from defrosting into the liquid refrigerant supply for said evaporators, means restrictively connecting said chamber means to the suction side of said compressor for controllably returning gaseous refrigerant from said chamber means to said compressor means, and valve means for affirmatively producing a reduced pressure in said chamber means relative to the pressure of said hot gaseous defrosting refrigerant for producing a pressure differential to cause the defrosting refrigerant to flow through the evaporators into said chAmber means.
 16. The refrigeration system of claim 15, wherein said valve means produces a predetermined and relatively low pressure in the liquid refrigerant supply.
 17. The refrigeration system of claim 15, wherein said valve means produces a predetermined pressure drop in said liquid refrigerant supply between said condenser-receiver and the location of communication of said chamber means.
 18. The refrigeration system of claim 15, wherein said valve means produces a predetermined pressure differential between said compressor discharge side and said chamber means and the absolute pressures vary with discharge side pressure variations.
 19. The refrigeration system of claim 15, wherein said valve means also affirmatively produces a like reduced pressure in said liquid refrigerant supply upstream of the location of said chamber means, and flash gas created by said liquid refrigerant pressure reduction is separated in said chamber means and also removed by said restrictive connecting means in a controlled manner to said compressor.
 20. In a closed cycle refrigeration system employing a compressor, condenser and receiver for supplying liquid refrigerant and multiple evaporators for multiple refrigerated fixtures wherein hot gas defrosting of such fixtures is provided, the improvement comprising, a vessel having chamber means for separating liquid and gaseous refrigerant, valve and conduit means connecting said evaporators to said vessel for selectively connecting it at least one of sad evaporators between the high pressure side of the system and said vessel to supply hot gaseous refrigerant to said evaporator for defrosting and for conducting the resultant liquid and gaseous refrigerant directly to said chamber means for separating the phases, said vessel having means in communication with the normal supply of liquid refrigerant to said evaporators for returning the liquid refrigerant resulting from defrosting into the liquid refrigerant supply for said evaporators, and means restrictively connecting said chamber means to the suction side of said compressor for controllably returning gaseous refrigerant from said chamber means to said compressor means.
 21. The refrigeration system improvement of claim 20 wherein a regulating valve means is provided in communication with said chamber means and affirmatively produces a pressure differential between said chamber means and the supply of hot gaseous defrosting refrigerant to cause the defrosting refrigerant to flow through the evaporators to said chamber means.
 22. The refrigeration system improvement of claim 20 wherein float control means are provided in said chamber means for maintaining a desired liquid level in said chamber means.
 23. The refrigeration system improvement of claim 20 wherein pressure regulating valve means are provided for controlling to three predetermined pressure levels the liquid refrigerant supply, the hot gaseous defrosting refrigerant supply and the resultant refrigerant condensate returning from defrosting.
 24. The refrigeration system improvement of claim 21 wherein valve and conduit means also serve to activate said regulating valve means only during the defrosting of an evaporator.
 25. The refrigeration system improvement of claim 20 wherein pressure regulating valve means are provided for producing a predetermined and constant low pressure in both the normal supply of liquid refrigerant and said vessel, and the refrigerant at said constant low pressure has a saturation temperature below the normal room temperature.
 26. The refrigeration system improvement of claim 20 wherein two pressure regulating valve means are provided for maintaining the pressure of said hot gaseous refrigerant within a small predetermined range under all ambient and operating conditions. 